Laser interferometers are widely used to measure a distance. Laser interferometers operate by splitting radiation source into two beams, an object beam and a reference beam, using, for example, a beam splitter. The reference beam is directed to a reference mirror located at a known distance from the beam splitter. The object beam is directed to a surface of a target located at some unknown distance from the beam splitter. The object beam is reflected from the surface, and is recombined with the reference beam.
The object beam and reference beam constructively or destructively interfere, depending on the relative phase of the object beam compared to the reference beam. A detector measures the intensity of the interference light related to the phase difference between the object beam and the reference beam, which is, in turn, related to the difference in the object path length from the beam splitter to the surface.
The periodic nature of interferometric output signal presents a challenge in absolute measurements. For example, some laser interferometer systems for determining an absolute distance to an object use multiple wavelengths to eliminate uncertainty regarding which specific period of the cyclic interference of light corresponds to a current measurement. However, those systems generally may not allow accuracy substantially better than a wavelength.
However, it is difficult to produce an optical signal with high specified frequency. This is because a laser for generating such signal has to be calibrated to the desired accuracy. The calibration requires stabilization against fluctuations of the temperature, vibrations and other possible disturbances. Such calibration is proved to be difficult in practice. In addition, modern detection systems are generally incapable of accurately measuring the frequency of optical signals to better than ±100 MHz. This means for a centimeter-scale gap, the measurement error is on the order of 10 nm, or a significant fraction of an optical wavelength. As a result, measuring distances with a specified accuracy poses a great challenge in the art of interferometric measurements.